On macOS, terminal(background=true) silently failed: the process returned a
session_id and exit_code=0 but the command never ran (empty stdout, no side
effects). Root cause is two interacting issues:
1. _find_shell was aliased to _find_bash, which prefers `shutil.which("bash")`
→ /bin/bash (GNU bash 3.2, still shipped on macOS) over $SHELL (/bin/zsh).
2. process_registry.spawn_local runs [shell, "-lic", "set +m; <cmd>"] with
stdin=/dev/null. bash 3.2 as a login shell sources ~/.bash_profile, which on
many macOS setups contains `exec /bin/zsh -l`; that exec replaces bash but
drops the -c argument, so the command is swallowed (exit 0, no output).
Decouple _find_shell from _find_bash: _find_shell now prefers the user's
configured $SHELL on POSIX (the shell they actually log in with), falling back
to _find_bash when $SHELL is unset/missing. _find_bash is unchanged, so callers
that genuinely need bash (e.g. the _run_bash login-shell snapshot) keep bash
semantics. zsh handles -lic correctly even with redirected stdin.
Salvaged from #42219 by @liuhao1024 (authorship preserved via cherry-pick).
On top of the original (8 unit tests covering $SHELL-set/unset/missing/empty,
Windows-ignores-$SHELL, _find_bash-unchanged), added an E2E regression test
that reproduces the real bash-3.2 login-shell swallow (exit 0 / no file) and
asserts the shell _find_shell selects actually executes a -lic background
command. Mutation-verified: reverting _find_shell to the bash alias fails the
$SHELL-preference test. Bug reproduced directly: /bin/bash 3.2 -lic with a
.bash_profile->exec-zsh creates no file; zsh -lic does.
Closes#42203. Supersedes #42290.
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